'tagName' is null or not an object in IE8. not validating radio buttons in IE8

Hi,
I am using validatious-custom-0.9.1.js framework in my project and I am trying to validate required fields for radio buttons. But it is not validating the radio buttons in IE8. I am getting the following error in console
Message: 'tagName' is null or not an object
Line: 7
Char: 9921
Code: 0
URI: http://validatious.org/design/js/val....js?1256063644

HTML code used.

Radio List
some text

Radio1

JS where it is failing - First 2 are the examples that are given in the framework file. So you will get to know how it is implemented

/* A radio button field is assumed to be either a list - ordered or unordered - with some element in front that acts as a label. This may be any element. If it is not in a list (ie the element does not have "li" parent elements), the label is assumed to be the element before the first input element.